The long range goal of this project is to understand the molecular basis for control of Schwann cell proliferation and differentiation. Schwann cell division during development ensures that sufficient Schwann cells are produced to ensheath and myelinate axons. The stimulus for Schwann cell division is provided by heregulin family growth factors produced by peripheral neurons. Heregulin stimulation of Schwann cells leads to phosphorylation of erbB2 and erbB3 heregulin receptors and activation of the MAP kinases ERK1 and ERK2. Schwann cell proliferation in response to heregulin, however, requires co-stimulation with an agent that raises intracellular cAMP levels, such as forskolin. The goal of the proposed studies is to elucidate the mechanism by which heregulin and cAMP coordinately regulate Schwann cell division. Forskolin has no effect on receptor phosphorylation or MAP kinase activation in response to heregulin, but is required for expression of cyclin D. Cyclin D expression is a critical determinant of commitment to cell division in response to mitogenic stimulation. Heregulin and forskolin synergistically activate transcription of the cyclin D3 gene ins Schwann cells. cAMP-dependent regulation of transcription is mediated by cAMP response elements (CREs) in the promoters of target genes, and activated through phosphorylation of CRE-binding protein CREB. The rat cyclin D3 gene contains a functional CRE. The principal CREB kinase in Schwann cells is activated by heregulin, but sustained accumulation of phospho-CREB requires co-stimulation with forskolin. The proposed work will explore the hypothesis that the function of cAMP is to serve as a co-activator of heregulin-dependent transcription of critical genes, such as cyclin D, through regulation of phospho-CREB levels. The specific aims are to 1) identify the DNA elements in the cyclin D3 promoter that are required for synergistic transcription activation by heregulin and cAMP, 2) to elucidate the molecular pathways by which cAMP and heregulin coordinately regulate phospho-CREB levels, and 3) to determine how perturbation of the mechanisms that regulate phospho-CREB levels affect cyclin D transcription and Schwann cell proliferation. Knowledge of the molecular mechanisms that regulate Schwann cell proliferation will facilitate the ability to modulate Schwann activity in injured and diseased nerves.